Water-cooled nuclear reactors, especially pressurized-water nuclear reactors, have fuel assemblies consisting of a bundle of fuel rods of great length arranged parallel to one another and held in a framework formed by guide tubes, spacers and two end connectors. The guide tubes are arranged in the longitudinal direction of the assembly and are connected to transverse spacers uniformly spaced over the length of the assembly.
The guide tubes are likewise connected, at each of their ends, to one or the other of two connectors forming pieces for stiffening and closing the assembly.
The fuel rods of the assembly form a bundle, in which the rods are parallel to one another and, in the cross-sections of the assembly, are arranged according to a regular network determined by the spacers. Some positions of the network are occupied by guide tubes which are generally connected rigidly to the spacers.
The guide tubes are longer than the fuel rods and are arranged in the bundle so as to have a part projecting relative to the bundle of fuel rods at each of their ends. The connectors are fastened to these projecting parts of the guide tubes, in order to ensure that the assembly is closed at each of its ends.
The fuel rods consist of sintered pellets of nuclear fuel material which are stacked inside a metal sheath isolating the pellets from the fluid surrounding the fuel assembly. If the sheath of a rod of a fuel assembly breaks, it is necessary to replace this rod very quickly in order to prevent any radioactive product from leaking into the cooling fluid of the reactor. To gain access to the fuel rods and carry out their replacement, one of the connectors of the assembly has to be removed, and this requires breaking the connections between the corresponding ends of the guide tubes and the connector.
The connectors have passage holes which reproduce the network of guide tubes and in each of which a guide tube is engaged and fastened.
In the most customary technique, the guide tubes are fastened permanently to the connectors, usually by welding, and it is impossible to carry out the removal of the connector in order to replace a rod. If a sheath on a rod breaks, therefore, the defective fuel assembly has to be replaced by a new fuel assembly. The defective fuel assembly is dismantled and the non-defective, but highly irradiated fuel rods contained in this assembly are recovered and reinstalled in a new fuel-assembly framework. To carry out these operations and manipulations of contaminated elements, it is necessary to take precautions and work in complex and costly installations. The recovery of the non-spent fuel rods in the defective assemblies is therefore an operation involving a very high outlay.
New fuel assemblies having guide tubes, of which the connection to at least one of the end connectors is removable, have therefore been designed and developed.
The assembly is placed under water in the vertical position, in a pool, such as a fuel-assembly storage pool, in which the replacement of the defective fuel rods is carried out. The guide tubes of the assembly are in vertical position, the assembly resting on the bottom of the pool by means of the lower connector. The upper connector is accessible, under a certain depth of water, from the top of the pool. Those parts of the guide tubes engaged in the upper connector of the assembly have a radially expandable part which, for example, can be attached to the end of the guide tube. This radially expandable part consists of a split bush with a radially projecting part which is intended to come to rest in a cavity of corresponding form machined inside the connector, in the passage hole of the guide tube. A blocking sleeve introduced inside the guide tube ensures the radial expansion of the split bush and the retention of the guide tube.
The removal of the upper connector of the assembly makes it necessary to remove the connection of each of the guide tubes, and this can be carried out by extracting the blocking sleeve, accessible from inside, from the top of the upper connector.
There are therefore devices making it possible to carry out the extraction of a blocking sleeve from a guide tube in an upper connector of a fuel assembly placed in vertical position in a pool. There are also devices for putting a blocking sleeve in place in a guide tube during the reinstallation of the upper connector of the assembly.
In particular, there is a known device for extracting the blocking sleeves from the guide tubes of a removable fuel assembly, which comprises a tubular element having slots and being radially expandable by means of a rod equipped with an end knob and mounted movably inside the tubular element. The tubular element has an annular rim projecting radially outwards. The tool incorporating the expandable tubular element is mounted on a centering plate which is placed on the upper connector of the assembly, thus making it possible to introduce the tubular element into a guide tube equipped internally with a blocking sleeve. An adjustable stop makes it possible to adjust the length of the tubular element introduced into the tube, in such a way that the projecting rim assumes a position just below the blocking sleeve. The movement of the rod makes it possible to expand the tubular element and bring the rim to bear on the lower part of the blocking sleeve.
The blocking sleeve is released and extracted by subjecting the upper part of the tool to repeated shocks.
This device is difficult to put into practice, because it requires the fitting of a centering plate and then an adjustment of the stop for adjusting the depth of insertion, and these operations have to be conducted under water inside a pool. In particular, it is difficult to ensure that the projecting rim of the expandable element is positioned very precisely under the blocking sleeve.
Moreover, since extraction is carried out by means of shocks, the successive jolts experienced by the tube and the upper connector of the assembly can cause some damage. At all events, the extraction operation is relative lengthy, inasmuch as, for each assembly, twenty four blocking sleeves, each arranged in one of the twenty four guide tubes, have to be withdrawn in succession before the removal of the upper connector is carried out.
Also known are other devices which are based on the same principle and in which the extraction is carried out as a result of the movement of a plate to which the expandable tubular element of the tool is fastened by means of a screw/nut device. The tool comprises a second plate coming to bear on the upper connector of the assembly, with the result that the reaction of the extraction forces on the bosses of the upper connector risks causing deformations which make it necessary to replace it in order to conduct a correct reinstallation of the fuel assembly.
Moreover, in all these devices, the extracted sleeve remains engaged on the extraction spindle, and it is therefore impossible to re-use the tool without separating the sleeve from the spindle. To conduct this operation of separating the sleeve, the tool has to be raised to the level of the service platform of the pool. This results in losses of time caused by the movements of the tool in order to raise it and lower it in the pool.